The Harvard Twin Study, using data collected from the Vietnam Era Twin (VET) Registry, demonstrated that vulnerability to alcohol abuse is significantly genetically influenced. The next step is to identify biological and psychological factors that mediate this genetic influence and characterize their mechanism(s). This application is a revision of our previous application. Although the high-risk design proposed in our original application could provide a great deal of information, we have concluded that a full twin design, using both members from twin pairs, will yield considerably more information. The adoption of a traditional twin design has eliminated a number of problems identified in our earlier proposal. We believe that our revised application with additional vulnerability indicators and a full twin design dramatically expands the scope of the project without dramatically expanding the cost. We will now be able to address the genetic and environmental determinants of vulnerability and will include several new and highly interesting domains. We will randomly select 400 twin pairs from among 1356 twin pairs interviewed in our previous study who did not serve in Vietnam during the war. The prevalence of alcohol abuse and dependence in our sample is high enough to provide good power using a random sample. Participants will be evaluated using putative vulnerability indicators, selected from the following domains: a) neuropsychological functioning; b) psychophysiological functioning; c) psychopathology; and d) personality and behavior. Data will be gathered from 200 MZ and 200 DZ twin pairs. Twins will be transported to participating sites at Boston, St. Louis; or Davis, CA to be evaluated with a full personality, psychological, psychophysiological, and neuropsychological protocol. The specific aims of the proposed study are: 1) to determine which of the putative vulnerability indicators are associated with alcohol abuse within individuals; 2) to determine the extent to which identified vulnerability indicators are influenced by genetic factors, environmental experiences shared by twins, and non-shared environmental experiences; 3) to determine the extent to which identified vulnerability indicators and alcohol abuse correlate because of genetic and/or environmental factors that influence both; 4) to distinguish alcohol abuse consequences from alcohol abuse vulnerability; and 5) to replicate and validate findings on definitions of alcoholism that may identify subtypes of alcohol abuse that are due primarily to genetic influences and subtypes that are due primarily to environmental influences (in conjunction with intramural researchers at NIDA).